For remote phosphor applications, i.e. applications in which phosphor and the light source, for example an LED, laser or laser diode, are spatially separate, thin phosphor layers are applied to surfaces (substrate), mechanically fixed with binders and bonded (air, immersion, etc.) to an optical system (lenses, collimators, etc.).
The use of binders is necessary, since the phosphors, which are usually in powder form, do not form mechanically stable layers, i.e. abrasion- and scratch-resistant layers, without the additional use of binders.
A problem with the use of binders for layer formation or stabilization is that these binders may interact with the phosphors and thereby adversely influence optical and thermal characteristics. Moreover, the binders must themselves be thermally and optically stable and not show any aging characteristics. For this reason, the use of an inert, optically transparent, thermally and optically stable binder is a precondition for the creation of stable and more durable phosphor layers.
The phosphors are excited to emit by light sources, for example blue diode lasers, of high power density (several W/mm2). The phosphor region may comprise one or more phosphors that are capable of at least partially transforming incident laser light or excitation light into wavelengths of transformed or converted light. The high thermal losses (Stokes) occurring in the conversion lead to heat being introduced into the phosphor layer. If these temperatures become too high, for example due to insufficient heat removal, thermally induced changes to the optical characteristics (emission wavelength, conversion efficiency, thermal quenching, etc.) may occur, or ultimately the destruction of the phosphors or the layer itself. This degeneration of the phosphor layer may be caused both by the phosphor and the binder. For this reason, the phosphor layer should be made such that heat can be optimally removed from it, in order to avoid the thermal destruction of the phosphors and the binder.
At present, silicones are used as binder matrices for optical excitation (for example LEDs). These do not allow excessive light power outputs (power densities of several W/mm2) or necessitate further technological expenditure (for example phosphor-coated rotating color wheels to reduce the light exposure time).
The coating process is limited by the type of the substrate materials. For instance, high-temperature processes are inconceivable on many plastics and metallic materials (for example aluminum) on account of their melting temperatures. On the other hand, alternatively available ceramic materials with good thermal conductivity (for example AlN) involve increased technological and financial expenditure. The known phosphor/silicone mixtures are usually applied directly to metallic substrates. These metallic substrates may additionally have a cooling function.
One object of the present invention is therefore to provide a method for producing phosphor layers that overcomes the known disadvantages and makes it possible to produce mechanically and thermally stable phosphor layers.